Pydantic AI Updates: May 29, 2026

1. Pydantic AI v1.104.0 adds Claude Opus 4.8 and fixes hybrid-route thinking forwarding

Pydantic AI. Pydantic AI v1.104.0 adds support for the newly released Claude Opus 4.8 model. The release also fixes a bug where the thinking=False parameter was not being forwarded on hybrid routes across OpenRouter, xAI, and Bedrock, aligning the silent-drop behavior of hybrid routes with direct routes. A separate fix preserves the tool_choice cache in single-tool scenarios on Bedrock. Source

2. Pydantic AI v2.0.0b4 turns the prepare-callback None deprecation into a hard error

Pydantic AI. The fourth v2.0 beta makes the most notable breaking change in the v2 line so far: prepare callbacks that return None now raise a TypeError rather than emitting the V1-era deprecation warning. The beta also rolls up the recent v1 work, including list_prompts and get_prompt on McpServer, message-timestamp round-tripping through VercelAIAdapter, Claude Opus 4.8 support, and the Bedrock error-mapping and UI file-handling fixes. The release signals the v2 surface is hardening as the breaking changes that were warnings in v1 become enforced. Source